The invention relates to thermoplastic fiber reinforced composites utilized in molding and extrusion, and in particular, composites having as a matrix resin polyamide polymers.
There has been growing interest in the use of long fiber reinforced thermoplastic composites as materials of construction because of the excellent physical properties inherent in a reinforcing network of fibrous materials such as glass, carbon, boron and alumina fibers embedded in a thermoplastic, as opposed to a thermosetting polymer matrix. Improvements in thermoplastic pultrusion techniques have occurred since the early 80""s for directly impregnating thermoplastic high polymers with a high degree of wet-but of continuous fiber rovings so that chopped pellets of 10 mm and longer can be used in let downs with thermoplastic resin compounds. Chopped pellets of good quality now can readily incorporated in molding compounds which are fed directly into extruders feeding injection molds or extrusion dies.
Long fiber reinforced composites of high temperature engineering resins, e.g., polyarylethers, especially polyetherketones and polyethersulphones are also known. For example, carbon fiber reinforced polycarbonate, and polyetheretherketone composite materials have found increasing commercial application. Polypropylene and nylon have generated the highest commercial volumes to date among the thermoplastics, especially directed to automotive end-uses.
Resin structures reinforced with long ( greater than 5 mm) fibers have superior processing properties and often better physical properties at the same degree of fiber loading, as compared to those reinforced with short (2 mm) fibers. The long fiber compounds are generally manufactured by a so-called pultrusion method in which a continuous reinforcing fiber bundle is impregnated with a thermoplastic resin while pulling fiber roving bundles through a cross-head extruder and then through a die (U.S. Pat. No. 3,993,726), or a continuous reinforcing fiber bundle is immersed in a thermoplastic resin melt to wet it while it is drawn and then pulled through a die.
The following patents relate to the formation of long fiber reinforced pultrusion composites: U.S. Pat. Nos. 4,541,884, 4,549,920, 4,559,262, 4,892,600, 5,019,450, 5,213,889.
U.S. Pat. No. 3,993,726 describes a process for the continuous production of articles of thermoplastic resin reinforced with long fibers of glass. According to the method described, the roving is impregnated with a mixture of wetting thermoplastic resin and reinforcing thermoplastic resin in a crosshead die fed by an extruder. The fibers are impregnated by first expanding the fibers of the roving and coating the expanded fibers in a crosshead die and passing the fibers and the thermoplastic mixture through a bar zone so that the material thoroughly penetrates into each fiber. This system is not completely satisfactory, since the wetting polymer dilutes in the other polymer and physical properties are lost.
U.S. Pat. No. 4,937,208 discloses a process for producing thermoplastic resins reinforced with long fibers in which rovings are impregnated with a wetting thermoplastic polymer by means of a die and a baffle zone, then taking up the impregnated rovings in a second die where they are then covered with a thermoplastic polymer. As explained, the wetting thermoplastic polymer must be compatible with the coating thermoplastic polymer. Suggested combinations disclosed were grafted high-density polyethylenexe2x80x94low-density polyethylene, grafted high-density polyethylenexe2x80x94ethylene-vinyl acetate (EVA) copolymer, grafted EVAxe2x80x94polyvinyl chloride, grafted polypropylenexe2x80x94polypropylene, sequenced poly(ether-amide) copolymerxe2x80x94polyamide functonalized polyamide (for example xcex1,xcexa9-diamines, xcex1,xcexa9-diacids, xcex1-amine-xcexa9-acid, monoamine)xe2x80x94polyamide, modified EVAxe2x80x94EVA or low-density polyethylene (PEbd).
U.S. Pat. No. 4,783,349 discloses a method of producing a fiber reinforced structure by melt impregnation of continuous fibers with a thermoformable polymer melt comprising pre-wetting the filaments of the fibers with a composition containing a melt plasticizer for the thermoformable polymer, prior to introducing the pre-wetted fibers into the melt, optionally removing the plasticizer when the composition contains a polymer miscible with plasticizer to leave polymer coated filaments and introducing the pre-wetted filaments into a polymer melt, optionally a polymer melt containing a metal plasticizer for the melt and preferably removing the plasticizer from the composition by volatilization. The pre-wetted polymer coated filaments can be let down into a compounding extruder running with the primary thermoplastic or processed directly in an injection molding apparatus being fed with a blend of fiber-reinforced, and non fiber-reinforced compound for use in processes other than direct melt impregnation. They can be used in the process known as film stacking impregnation or in processes involving initial impregnation using particulate polymers or alternatively in conventional extrusion compounding.
Nylon polymers have also been stabilized by incorporating stabilizing materials directly into the polymer chain. For example, epsilon-caprolactam may be polymerized in the presence of water, carboxylic acids and hindered piperidine derivatives (polyalkylpiperidines) to form a modified nylon 6 polymer that is stabilized against heat and light degradation. Such a stabilized polymer is described in PCT Application WO 95/28443 published Oct. 26, 1995.
Difficulties in the pultrusion of engineering resins, e.g. polyamide, result from the poor wet-out of the fibers. This problem is not so severe in the case of polypropylene. Some approaches to improving the pultrusion of polypropylene have been disclosed whereby the reinforcing fiber roving is treated in a separate step with a wetting thermoplastic polymer or solution thereof, such as a with a polyolefin modified by grafting with an ethylenically unsaturated polycarboxylic acid. However, this method requires a selection of specialized wetting polymers which must be substantially compatible with the matrix polymer, as well as a limit in the highest flexural or tensile modulus, generally, not to mention the cost and environmental impact of such a method. Recently, higher melt-flow polypropylene products have become the standard material for the one-pass pultrusion at high (30 vol. %) fiber loadings.
For a variety of reasons, such as the need to reduce costs and to fabricate lighter structures, improved flexural and tensile modulus are desired from less costly polymer composites. The limitations of flowability for fiber-reinforced composites present problems particularly in the injection molding of large surface area articles using a composite wherein the fiber content exceeds 15, 20 and even greater than 30 volume percent. Desirable thermoplastic materials, such as polyamides, in particular the phthalamide-types which otherwise provide inherently high modulus, and physical properties at high in-service temperatures have limits on moldability, especially in light of the molding tonnage available to molders. The high volume content of fibers results in relatively little polymer being available at the surfaces of the work pieces to be joined. Differences in the dispersion patterns of the long fibers results in variations in resulting physical properties of the molded composite.
In the manufacture of long fiber reinforced pellets of conventional engineering resins having acceptable end-use physical properties have melt viscosities in excess of 100 Ns/m2. Adequate wetting of the fibers in a pultrusion process with a melt of such high viscosity is not easily achieved. Problems of fiber roving breakage, lowering of line speeds to promote wet-out, and polymer degradation are possible. Any attempts to improve production by reducing the pultrusion matrix polymer melt viscosity, such as by increasing the melt temperature runs a greater risk of operating in an unstable thermal window. Other methods to reduce melt viscosity of the pultrusion matrix polymer by blending the matrix polymer with higher melt-flow materials is accompanied by undesired loss in physical properties, greater complexity and/or cost.
For molding or extrusion of shaped articles from long-fiber reinforced composites in which a fiber-reinforced concentrate is let down with additional matrix resin compound, there is a second added work history experienced by the fiber reinforced concentrate. This results inevitably in fiber breakage (attrition) and an inherent loss in flexural and tensile moduli, and/or impact strength. Improvements in the resulting physical properties of the final composite are sought.
It would be desirable to improve the physical properties of long fiber reinforced thermoplastic composites made by the method of letting down long fiber concentrates wihout resorting to harsh conditions, or blending with polymeric flow improvers therefore.
The invention provides a process for making a letdown of glass fiber reinforced composite comprising a polymer matrix and from 5 wt. % to about 60 wt. % of reinforcing fibers long fibers. The fibers are incorporated in the let-down as a concentrate of pultruded composite of substantially parallel fibers and thermoplastic polyamide where fiber length equals pellet length. The fibers in the let-down have improved dispersion and surprisingly greater, as opposed to less orientation in the final thermoplastic polymer matrix relative to prior art long fiber composite, thereby providing substantially improved physical properties from the forming of the fiber reinforced pellet with a modified pellet polymer matrix. The long fibers are incorporated as a concentrate pellet in the final matrix polymer by mixing pellets with the final polymer compound in a heated extrusion mixing zone used to convey a composite melt into a shaping zone, whereby the mixing under heat and rotational shear disperses the long fiber-reinforced pellets into the let-down matrix polymer.
The long fiber reinforced pellets comprise a modified pellet polymer matrix according to (a) or (b),
(a) the pellet matrix polymer is modified in the polymer polymerizing process by introducing an effective amount of a compound according to (A)-(I) or where applicable, a hydrolyzate thereof, with precursors of the matrix polymer, and method
(B) whereby the long fiber pellet is formed with a pellet matrix polymer that contains an effective amount of compounds selected from (B)xe2x80x94(XX) in intimate contact with the continuous fiber during the formation of the long fiber pellets.